To healthcheck the "foo" plugin:
    :CheckHealth foo
To healthcheck the "foo" and "bar" plugins:
    :CheckHealth foo bar
To run all auto-discovered healthchecks:
    :CheckHealth

- Overlay markdown syntax/filetype, don't invent new filetypes/syntaxes.
- migrate s:check_ruby()
- s:indent_after_line1
- Less-verbose output

- Use execute() instead of redir
- Fixed logic on suboptimal pyenv/virtualenv checks.
- Move system calls from strings to lists. Fixes #5218
- Add highlighting
- Automatically discover health checkers
- Add tests

Helped-by: Shougo Matsushita <Shougo.Matsu@gmail.com>
Helped-by: Tommy Allen <tommy@esdf.io>

Closes #4932

This makes stderr and exit callbacks work for rpc jobs

Without the for-loop it is easier to follow, more explicit, and fewer
lines.

Uses $NVIM_RPLUGIN_MANIFEST if available

- Weird tab+space combination used for alignment. All spaces now
- Added back <C-T> mapping (somehow we missed that completely)
- Fixed mistake that <Plug>(Man) opens in a new tab. Also added note at
  top on how the window is chosen/opened.
- Clarified q local mapping
- Removed section that shows an example autocmd to add desired folding
  style.
- Removed random line in `usr_12.txt` about `<Leader>` and backslash.
- :Man supports completion, not auto-completion.

Closes #5171

In 3b12bb225adda2aac40a55f7009cae05311b2a43, ":oldfiles" was taught to
behave like Vim's ":browse oldfiles" if ":oldfiles!" was used.  However,
this conflates the use of ! for abandoning a modified buffer with
choosing one file out of a list of oldfiles.

Now that ":browse" is supported again, ":browse oldfiles" will allow the
user to select an old file, while still complaining if that would cause
a modified buffer to be abandoned.  ":browse oldfiles!" will just
abandon the buffer, as expected.